PHYSICS OF SEMICONDUCTOR DEVICES JNTU SEMESTER 2 ENGINEERING PHYSICS 2

1. Physics of
Semiconductor Devices

2. Formation of PN – Junction
When a P-type Semiconductor is joined together
with an N-type Semiconductor a PN junction is
formed. And it is also known as a Semiconductor
Diode.
Semiconductor diodes are widely used in
Rectifiers which converts input AC signal into
DC output signal.

3. P N
-
-
-
-
-
-
-
- +
+
+
+
+
+
+
+
Space charge region
(OR)
Depletion region
JunctionJunction
Ionized acceptorsIonized acceptors Ionized donorsIonized donors
Potential barrier(VPotential barrier(V00))
Potential barrier widthPotential barrier width
(W)(W)

4. Depletion Region & Space Charge
The diffusing majority carriers from the two regions
recombine near the junction and disappear.
The uncompensated Acceptor and Donor ions set up an
Electric field which halts majority carrier Diffusion and
causes minority carrier Drift.
The two kinds of majority carriers diffusing across the
junction meet each other near the junction and undergo
recombination's, leaving negative ions on the P-side and
positive ions on the N-side of the junction.
This distribution of Positive and Negative Charges is
called Space charge.

5. N P
Cathode Anode
_
+
Diode Symbol

6. Fermi level
Depletion region
PN - junction
EFn
Valence band
Conduction band
NP
EFp
E
eVB
eVB
Ev
Ec
Ev
Ec
Energy level diagram

7. V - I Characteristics of PN Junction
The diode can be operated in two different ways, as Forward
and Reverse bias.
When positive terminal of the battery is connected to the P-
type & negative terminal is to the N-type of the PN-junction
diode, known the diode is kept in forward bias.
When negative terminal of the battery is connected to the P-
type & positive terminal is to the N-type of the PN-junction
diode, known the diode is kept in reverse bias.

8. Depletion Layer
N-regionP-region
+
+
+
+
Donor ionsAcceptor ions
V
Potential barrier
Forward bias

9. Depletion Layer
N-regionP-region
+
+
+
+
Donor ionsAcceptor ions
V
Potential barrier
Reverse bias

10. Forward Bias
Reverse Bias V
I
Current
Reverse
break
down
current
Forward Current
Knee Voltage

11.  The region between knee voltage & breakdown
voltage is known as non-ohmic region.
 Above the knee & breakdown voltage the current
increases.
 Breakdown voltage is due to thermally broken
covalent bonds.
 Diode is conducting in forward bias &
non-conducting in reverse bias.

12. Light emitting diodes
LED’ s are the most visible type of diode, that emits a
fairly visible colored light, invisible infra-red or laser
type light when a forward current is passed through
them.
Principle:
Basically LED are made from a very thin layer of fairly
heavily doped semiconductor material.
When the diode is forward biased, electrons from
conduction band combine with holes from in valence
band, releasing sufficient energy to produce photons of
light.

13. Cathode
λ
υ
c
hE
hE
=
=

14. V
Current

15. A Rectifier is an electronic circuit which
converts alternating current to direct current
(OR) unidirectional current.
Rectifiers are mainly three types
1.Half wave rectifiers
2.Full wave rectifiers
3.Bridge rectifiers

16. An electronic circuit which converts
alternating voltage (OR) current for
half the period of input cycle hence
it is named as half-wave rectifier.

17. Half – Wave Rectifier
A.C Input
Pulsated
D .C Output
transformer
B
RL
rf

18. The ratio of D.C power output to applied A.C
power input is known as rectifier efficiency.

19. An electronic circuit which converts
alternating voltage (OR) current into
pulsating voltage (OR) current during
both half cycle of input is known as
full-wave rectifier.

20. Full Wave Rectifier
D .C Output
A
B
Center tapped
transformer
rf
rf
RL

21. The ratio of D.C power output to applied A.C
power input is known as rectifier efficiency.

22. 2
3
2
2
3
2
)
2
(2Where
)1().........exp(
)exp()
2
(2
h
kTm
N
kT
EE
Nn
kT
EE
h
kTm
n
e
C
cF
C
cFe
∗
∗
=
−
=
−
=
π
π
2
3
2
2
3
2
)
2
(2Where
)2).......(exp(
)exp()
2
(2
h
kTm
N
kT
EE
Np
kT
EE
h
kTm
p
h
v
FV
v
FVe
∗
∗
=
−
=
−
=
π
π
Consider Intrinsic Semiconductor
Electron Concentration Holes Concentration
ion 1 & 2 holds good for both intrinsic and extrinsic semicondu
r Thermal equilibrium condition.

23. )exp(
,,
kT
EE
Nn
EEEEnn
cnFn
Cn
cncFnFn
−
=
===
For N type Material
)exp(
,,
kT
EE
Nn
EEEEnn
cpFp
Cp
cpcFpFp
−
=
===
For P type Material
Electron Concentration
)3.().........exp(
)exp(
)exp(
)exp(
)exp(
kT
eV
nn
kT
eV
n
n
kT
EE
kT
EE
kT
EE
n
n
B
np
B
p
n
cncp
cpFp
cnFn
p
n
−
=
=
−
→
−
−
=

24. )exp(
,,
kT
EE
Np
EEEEpp
Fpvp
vp
vpvFpFp
−
=
===
For N type Material
)exp(
,,
kT
EE
Np
EEEEpp
Fnvn
vn
vnvFnFn
−
=
===
For P type Material
Hole Concentration
)4().........exp(
)exp(
)exp(
)exp(
)exp(
kT
eV
pp
kT
eV
p
p
kT
EE
kT
EE
kT
EE
p
p
B
pn
B
p
n
vpvn
Fpvp
Fnvn
p
n
−
=
−
=
−
→
−
−
=

25. }1){exp(
)exp(
)exp().exp(
)
)(
exp(
−=∆
=∆+
−
=∆+
−−
=∆+
kT
eV
nn
kT
eV
nnn
kT
eV
kT
eV
nnn
kT
VVe
nnn
pp
ppp
B
npp
B
npp

26. }1{11 −=∆= TK
eV
ppe
B
encnci
}1{22 −=∆= TK
eV
nnh
B
epcpci

27. }1){( 21 −+=+= TK
eV
nphe
B
epcnciiI
}1){( 21 −+=+=
−
TK
eV
nphe
B
epcnciiI

28. I0 is called reverse saturation current
021 )( IpcncI np −=+−=
1<<
−
KT
eV
e
1)exp(0 −
−
=
KT
eV
II

29. 1)exp(0 −
−
=
KT
eV
II
β